The present invention relates, in its more general aspect, to a superconducting cable having at least one superconducting layer.
The term xe2x80x9csuperconducting cablexe2x80x9d indicates an electrical transmission cable comprising at least an element of superconducting material.
The superconducting cable of the present invention may be a warm dielectric (WD) or cold dielectric (CD) cable. See, for example, Engelhardt J. S. et al., Application Consideration for HTSC Power Transmission Cable, 5th Annual Conference on Superconductivity and Application, Buffalo, N.Y., Sep. 24-26, 1991, FIG. 5 for a WD cable, and FIG. 6 for a CD cable.
A WD cable generally comprises one or more layers of tapes of superconducting material wound on a support, typically tubular, defining the cryogen fluid flow channel. Externally to the superconducting tapes a cryostat and an electric insulator are provided.
A CD cable generally comprises, in addition to the layers mentioned above for the WD cable, further layer(s) constituting the so-called return conductor and wound externally to the electric insulator and surrounded by a wall partially defining a second cryogen fluid flow channel.
The term xe2x80x9csuperconducting materialxe2x80x9d indicates a material, such as for example, special niobium-titanium alloys or ceramics based on mixed oxides of copper, barium and yttrium (YBCO) or gadolinium, samarium or other rare earth (REBCO), or of bismuth (BSCCO), or thallium and mercury, and lead, strontium, calcium, copper, comprising a superconducting phase having a substantially null resistivity below a given temperature, defined as critical temperature or Tc.
The operative temperature of a superconducting cable is equal or, preferably, lower than the Tc of the superconducting material present therein.
Usually the superconducting material, particularly the BSCCO material, is produced and used in form of tapes wherein the material is surrounded by a metal, generally silver optionally added with aluminium or magnesium, and preferably reinforced by a further layer of metal, for example stainless steel.
The tapes are usually helicoidally wound around a support, parallel each other, at a prefixed distance so as to form gaps among the tapes of each layer. The tapes are not wound in contact one another.
The term xe2x80x9csuperconducting conductorxe2x80x9d indicates the electrically active portion of a superconducting cable, comprising a support and at least one layer of superconducting material.
The term xe2x80x9csuperconducting layerxe2x80x9d indicates a layer of tapes of superconducting material wound around a support or around another superconducting layer or around a dielectric, this latter being the case of the CD cable. One or more layers of tapes of superconducting material may constitute a phase conductor or, in the case of a cold dielectric cable, a return conductor.
The Applicant has found that high gaps among tapes of superconducting material may produce damages to the tapes due to the pressure exerted on the tapes by the parts surrounding them and to the friction among the various components during the manufacturing and handling of the cable. Some kind of cables shows superconducting conductors with a restricted number of tapes per layer with respect to the circumference of their support. It is the case, for example, of a 3-tape conductor wherein the tapes are in wye configuration. Especially in the case of a CD cable, the pressure due to the weight of the cable itself is significant in view of the small area of the tape, of their structural fragility and of the sensitivity of the superconducting material contained therein. The problem is particularly evident at the contact points among tapes of two adjacent layers. Another instance regards the return conductor of a CD cable, which has to have the same current capacity (also known as ampacity) of the phase conductor. The most simple and economic way for attaining this goal is to use the same number of tapes per layer of the phase conductor. In view of the greater diameter of the return conductor (twice that of the phase conductor), the distance (gap) among the tapes of the same layer of this conductor is remarkably higher. For example, in a phase conductor having a diameter of 32 mm and 14 5-mm wide tapes per layer wound thereupon, the gap among the tapes is 2.14 mm wide for the first layer. The return conductor of said phase conductor, and the same number of 5-mm wide tapes per layer, has a diameter of 64 mm and the gap among the tapes is 9.35 mm wide for the first layer.
It has now been found that by partially filling the gaps among the tapes of superconducting material with a non-superconducting material mechanical damages to the tapes are avoided.
Therefore in a first aspect the present invention relates to a superconducting cable comprising at least one layer of tapes of superconducting material wound on a support at a prefixed distance so as gaps are formed among adjacent tapes, wherein a non-superconducting material is interposed between adjacent tapes to partially fill said gaps.
Preferably, the superconducting cable according to the invention is a cold dielectric cable comprising a phase conductor including at least a first layer of tapes of superconducting material wound on a support at a prefixed distance so as gaps are formed among adjacent tapes, and a return conductor including at least a second layer of tapes of superconducting material, as return conductor wound on a support at a prefixed distance so as gaps are formed among adjacent tapes, wherein the non-superconducting material is present at least among the tapes of superconducting material of the return conductor.
Specifically the non-superconducting material is in form of wires or, preferably, of tapes.
The non-superconducting material has preferably a thickness differing from that of the tapes of the superconducting material of an amount not higher than +/xe2x88x9215%, more preferably than +/xe2x88x9210%, even more preferably than +/xe2x88x925%. Preferably the thickness of the non-superconducting material is substantially equal to that of the tapes of superconducting material.
The non-superconducting material partially fills the gaps among the tapes of superconducting material. The necessity of maintaining a gap, even if reduced among tapes of superconducting material and non-superconducting material is due both to the head for HTS-tape laying employed for the deposition of the tapes, and the necessity of avoiding friction among the tapes and of undesirable electromagnetical phenomena. Specifically the width of the non-superconducting material is such that a gap of 0.1-3 mm remains between a tape of superconducting material and the adjacent tape or wire of non-superconducting material, more preferably of 0.1-2 mm.
The non-superconducting material may be a plastic, a metal or a combination thereof. When it is totally or partially metallic, the metal has amagnetic characteristics at the operative temperature of the cable. Preferred metals according to the invention are copper, silver and gold.
When the non-superconducting material is copper or silver or gold or alloys thereof, it shows the additional advantages of protecting the superconducting material against overcurrents generated in short-circuit events. A preferred metal according to the invention is copper.
The non-superconducting material is preferably longitudinally wound on the support or on the underlying superconducting layer, and alternated with the tapes of superconducting material.
A further aspect of the present invention relates to a method for minimizing mechanical stresses to tapes of superconducting material wound on a support at a prefixed distance so as gaps are formed among adjacent tapes, comprising the phase of interposing a non-superconducting material between adjacent tapes to partially fill said gaps.
In a further aspect, the present invention relates to a method for producing a superconducting conductor comprising at least one layer of superconducting tapes wound on a support at a prefixed distance so as gaps are formed among the tapes, wherein a non-superconducting material is interposed between adjacent tapes to partially fill said gaps.
Another further aspect of the present invention relates to current transmission/distribution network comprising at least one superconducting cable comprising at least one layer of tapes of superconducting material wound on a support at a prefixed distance such as gaps are formed among the tapes, wherein a non-superconducting material is interposed between adjacent tapes to partially fill said gaps.